It is sometimes necessary to expand the diameter of a hole previously bored in an object to be drilled. Such necessity arises widely in the case where a large-diameter hole is formed by using a large-size or special device in civil engineering and construction and also in the case where a relatively small-diameter hole is formed by using a general-purpose device in the machining of an assembled part.
As an example of the latter case, in model making, part machining of precision machinery, and the like, it is sometimes necessary that a hole be formed in a workpiece by using a lathe, milling machine, drilling machine, or the like, and then the diameter of the hole be expanded to form a large-diameter hole.
In this case, centering for causing the center of the hole bored previously in the workpiece and the center of the expanded hole, in other words, the center of the hole bored previously in the workpiece and the center of a drill for forming the expanded hole to coincide with each other is not necessarily easy to perform. It is needless to say that if the expanded hole is drilled in a decentering state, various troubles occur, for example, when the workpiece is assembled by using a screw or the like.
In the case where the large-diameter hole is formed, the trouble caused by decentering does not necessarily present a big problem as compared with the necessity for contrivance for surely performing the hole expanding work itself. In contrast, in the case where the small-diameter hole is formed, the trouble cannot be neglected.
For example, in model making, a process in which many workpieces formed of a metallic material are placed on a table in a lapped manner, and a through hole having a diameter of about several millimeters are drilled collectively in the workpieces is favorable for the subsequent work. Thereafter, the diameter of the previously bored hole is expanded by drilling the hole again to form a through hole having a different larger diameter, which is finally necessary for each workpiece, for each of the workpieces.
Also, when a countersinking process, in which the head of a fastening member such as a screw installed in the hole in the workpiece is embedded within the hole, is necessary, work for machining the open end of the previously bored hole into a tapered shape is performed separately. It can be said that this countersinking process is also one of jobs for expanding the diameter of the previously bored hole.
Needless to say, the work for expanding the diameter of the hole including the countersinking work may sometimes become necessary similarly even in the case where a non-penetrating bottomed hole is formed in one workpiece.
On the industrial base for handling workpieces in large quantities, the work for expanding the diameter of the hole can be performed easily and exactly by using an NC (numerically controlled) lathe capable of performing positioning etc. easily, or a hole having a desired diameter for each workpiece can also be bored directly with ease without performing boring work twice.
Also, as a method for further expanding a hole having a diameter as large as, for example, about several tens millimeters or more, a boring method using, for example, a core drill can be cited. In this case, to perform stable work, an auxiliary tool having a unique shape according to the feature of boring device is sometimes used (for example, refer to Patent Document 1).
Also, there have been disclosed a centering bar the tip end of which is tapered to perform centering by utilizing the taper (refer to Patent Document 2), a device to which a center pin having a taper-shaped tip end is attached to automatically cause the center of a previously bored hole and the center of the main spindle of finishing machine to mechanically coincide with each other (refer to Patent Document 3), and a centering tool engagingly fitted on one side of a socket the other side of which is attached detachably to the tool of a machine tool so that the head of a needle having a head of spherical shape etc. is turnable (refer to Patent Document 4).
Patent Document 1: Japanese Patent Laid-Open No. 2001
Patent Document 2: Microfilm of Japanese Utility Model Application No. 51-42802 (Japanese Utility Model Laid-Open No. 52-133399)
Patent Document 3: Microfilm of Japanese Utility Model Application No. 55-85018 (Japanese Utility Model Laid-Open No. 57-9434)
Patent Document 4: Microfilm of Japanese Utility Model Application No. 53-130177 (Japanese Utility Model Laid-Open No. 55-50709)
In the above-described model making etc. performed by an individual in the home as a hobby, holes are usually bored by using an inexpensive mini-lathe etc. Therefore, the work for drilling an expanded hole by performing centering is considerably troublesome. The same is true for the case where holes are bored by using the mini-lathe etc. in the work for machining and assembling precision parts of a relatively small lot. Such a problem is not necessarily solved sufficiently by the above-described related arts.
The present invention has been made in view of the above-described problems, and accordingly an object thereof is to provide an expanded hole drilling device capable of expanding the diameter of a previously bored hole to a larger diameter without causing decentering by using a simple method using a simple device, and a component for the expanded hole drilling device.